By default, .NET windows forms applications generally allow multiple instances of the same application to be started on the same machine. However, if two or more threads attempt to access a shared resource such as shared memory at the same time, concurrency issues may occur. And in a production environment, this might result in data inconsistency or inaccuracy.

This problem can be avoided using mutual exclusion (Mutex) to ensure that simultaneous instances of the same program cannot be run.

The code below demonstrates how this may be done.

static void Main()
{
   bool createdNew = true;
   using (Mutex mutex = new Mutex(true, "DemoApp", out createdNew))
   {
      if (createdNew)
      {
         Application.EnableVisualStyles();
         Application.SetCompatibleTextRenderingDefault(false);
         Application.Run(new Form1());
      }
      else
      {
         MessageBox.Show("The Application Is Already Running", "DemoApp", MessageBoxButtons.OK, MessageBoxIcon.Error);
      }
   }
}

The message shown in the picture above is displayed to the user if he attempts to start an instance of the program when another instance of the same program is already running.